1. Field of the Invention
The present invention relates to a circuit that detects a tone signal. Specifically, the invention relates to a tone signal detection circuit used in an FM receiver or an FM portable telephone.
2. Description of the Related Art
A communication system using an FM receiver, FM portable telephone, or the like employs a tone signal in order to determine if a call is permissible.
The operation of a conventional tone signal detection means is as follows. First, a bandpass filter extracts a signal of a desired tone frequency band from an input signal. The extracted signal of the desired tone frequency band is then converted into a signal of a level corresponding to the amplitude of the extracted signal. When the level is more than a predetermined threshold, the input signal is judged to contain the tone signal.
However, the foregoing conventional detection means cannot discriminate noise from the tone signal, in case there are noise components near the passband of the bandpass filter, especially in FM radio communication systems in which, as the reception electric field strength decreases, the white noises of the signal increase and the signal decreases. From such circumstances, a tone signal detection circuit has been expected which can accurately detect a tone signal regardless of the reception electric field strength.
It is therefore an object of the present invention to provide a tone signal detection circuit that accurately detects the tone signal regardless of the reception electric field strength.
In order to accomplish the foregoing object, the tone signal detection circuit according to the invention is comprised of: a first filter that extracts a signal of a specific frequency from an input signal, a second filter that excludes the signal of the specific frequency from the input signal and extracts signals of other frequencies near the specific frequency, a subtractor that subtracts an output of the second filter from an output of the first filter, and a judgment circuit that judges the presence of the tone signal in the input signal on the basis of an output from the subtractor.